This application claims foreign priority under 35 U.S.C. xc2xa7119 from both Italian Patent Application Serial Number MI99A 001241 filed Jun. 6, 1999, and Italian Patent Application Serial Number MI2000A 000475 filed Mar. 9, 2000, both of which are incorporated herein by reference for all purposes.
1. The Field of the Invention
The present invention relates to composite materials capable of selectively sorbing hydrogen without requiring an activation treatment, and to composite materials capable of sorbing hydrogen as well as other gases. The present invention also refers to methods for the production of such materials.
2. Background
In many technologically advanced applications gas sorption is achieved with non-evaporable getter (NEG) materials. NEG materials are most frequently found in two types of applications in particular. In the first type of application a NEG material is used to purify a gas stream by sorbing unwanted species. For example, in the semiconductor industry bothersome species such as hydrogen, oxygen, nitrogen, water, oxides of carbon, and oxides of nitrogen are removed from noble gas streams. Similarly, gases used in the manufacture of certain gas-filled devices such as light bulbs are similarly purified to provide advantages such as improve filament lifetimes.
In the second type of application a NEG material is used to maintain a high degree of vacuum within a sealed enclosure. Processing chambers are common examples of such enclosures in the semiconductor industry. Beyond the semiconductor industry such enclosures can be found in thermal insulation devices such as thermal bottles, dewars, and insulated pipes for oil extraction and for oil transport in arctic and undersea regions. Sealed enclosures for these applications typically consist of an inner wall and an outer wall with an evacuated volume maintained between the two walls. For oil extraction and transport it is frequently necessary to use insulated pipes in order to prevent excessive cooling of the fluid. Such cooling can cause the heavier components of the oil to solidify with a resulting increase in the total viscosity thereof, potentially creating a blockage.
NEG materials include metals such as zirconium and titanium and alloys based on these metals. Such alloys can include one or more other elements selected from amongst the transition metals and aluminum. NEG materials have been the subject of several patents. U.S. Pat. No. 3,203,901 describes Zrxe2x80x94Al alloys, and particularly an alloy whose weight percent composition is Zr 84%xe2x80x94Al 16%, produced and sold by SAES Getters S.p.A., Lainate, Italy, under the name St 101(copyright). U.S. Pat. No. 4,071,335 describes Zrxe2x80x94Ni alloys, and particularly an alloy whose weight composition is Zr 75.7%xe2x80x94Ni 24.3%, produced and sold by SAES Getters S.p.A., Lainate, Italy, under the name St 199(trademark). U.S. Pat. No. 4,306,887 describes Zrxe2x80x94Fe alloys, and particularly an alloy whose weight composition is Zr 76.6%xe2x80x94Fe 23.4%, produced and sold by SAES Getters S.p.A., Lainate, Italy, under the name St 198(trademark). U.S. Pat. No. 4,312,669 describes Zrxe2x80x94Vxe2x80x94Fe alloys, and particularly an alloy whose weight percent composition is Zr 70%-V 24.6%xe2x80x94Fe 5.4%, produced and sold under the name St 707(copyright). U.S. Pat. No. 4,668,424 describes Zrxe2x80x94Nixe2x80x94Axe2x80x94M alloys, where A represents one or more rare earth elements, and M represents one or more elements selected from amongst cobalt, copper, iron, aluminum, tin, titanium, silicon. Patent application EP-A-869,195 describes Zrxe2x80x94Coxe2x80x94A alloys, where A is an element selected from amongst yttrium, lanthanum, the rare earth elements, and mixtures thereof. This patent application particularly discloses an alloy whose weight percent composition is Zr 80.8%xe2x80x94Co 14.2%-A 5%, produced and sold by SAES Getters S.p.A., Lainate, Italy, under the name St 787(trademark). Finally, U.S. Pat. No. 4,457,891 describes Tixe2x80x94Ni and Tixe2x80x94Vxe2x80x94Mn alloys.
Patent EP-B-291,123 describes the use in lamps of getter materials having Zrxe2x80x94Pdxe2x80x94O compositions, where the palladium is present in molar concentrations between 0.4% and 15% and the molar ratio between oxygen and zirconium is within the range 0.02-1.
The sorption of gases by NEG materials occurs in two steps. The first step is the superficial chemisorption of the gaseous species onto the surface of the NEG material, generally accompanied by the dissociation of the species into its constituent atoms. In the second step the constituent atoms diffuse into the bulk of the NEG material. In the case of hydrogen sorption, as hydrogen atoms spread inside the material they first form solid solutions, even at low temperatures. As the hydrogen concentration increases, hydrides such as ZrH2 are formed. Therefore, the sorption capacity for hydrogen is high even at low temperatures.
This second step is different for elements such as oxygen, carbon and nitrogen. At relatively low temperatures (generally lower than about 300-500xc2x0 C. according to the type of the NEG material) only superficial chemisorption occurs and surface layers of oxide, carbide or nitride compounds are formed. These layers effectively block bulk diffusion from occurring. At higher temperatures the oxygen, nitrogen and carbon atoms are able to diffuse into the NEG material, thus regenerating a clean surface for further gas sorption. Therefore, surface cleaning can be achieved continuously by constantly maintaining a NEG material at a sufficiently high temperature. Alternately, the surface of a NEG material maintained at a low temperature can be cleaned by periodically bringing it to a sufficiently high temperature. This latter process is commonly known as an activation treatment, and may be carried out at regular intervals or when a loss of sorption capacity is observed.
However, there are many applications for NEG materials in which the working temperature is at or below room and activation treatments are practically impossible. Such applications include maintaining high vacuum levels in sealed enclosures like those found in thermal bottles, fluorescent lamps, and the insulated pipes used in oil extraction and transport. Another important application of this kind is in batteries, both of the rechargeable kind such as Ni-metal hydride batteries, and of the non rechargeable kind, such as conventional alkaline batteries. As is well known in the art, batteries include an anode, a cathode, and an electrolyte disposed between them, all contained within a casing. Both alkaline and rechargeable batteries, under certain operating conditions, may release hydrogen causing the casing to swell and creating a risk of explosion.
In these low-temperature applications the sorption of relatively small quantities of oxygen, nitrogen or carbon produces a passivating layer on the surface of the NEG material, as previously described, which prevents further gas sorption and reduces the material""s sorption capacity to a fraction of its theoretical value. Further, the passivating layer blocks hydrogen sorption which, as already explained, would otherwise occur to a high extent even at room temperature.
In some applications that employ NEG materials the presence of hydrogen can be especially harmful. In the case of thermal insulation applications, this is because hydrogen is the best thermal conductor amongst the various gases. Therefore, hydrogen in an evacuated volume, even in small quantities, notably worsens the thermal insulating property thereof. The presence of hydrogen in the gaseous filling mixture of lamps modifies the discharge conditions, and thus both prevents the lamp from functioning optimally and generally shortens its life. The presence of hydrogen is even more troublesome in the pipes used for oil extraction. In this application, acid solutions containing, for instance, hydrochloric acid, nitric acid, hydrofluoric acid, or mixtures thereof, are passed through the pipe in order to promote the disintegration of rocks wherefrom oil is extracted. However, these acids can cause corrosion of the pipes, forming microperforations and while generating large quantities of hydrogen. The hydrogen then may easily pass through the microperforations and into the surrounding evacuated volume, serious degrading its thermal insulation properties.
Improved hydrogen sorption by getter materials is the subject of international patent application WO 98/37958 and patent SU-A-1,141,920, which both describe coating NEG materials with palladium. According to these documents, the coating is performed by sputtering to obtain a continuous hydrogen-permeable film of palladium metal across the surface of the NEG material. In use, the getter material contacts an evacuated volume or a gas to be purified only through this continuous film. Further, getter devices created through the sputtering processes described in these documents can only be made in a planar configuration, which is not suitable for many possible applications. Additionally, the gas sorption capacity (i.e., the maximum gas quantity which can be sorbed) of these systems is low because of the lesser quantity of NEG material in the planar deposits. Lastly, the described devices are totally selective towards hydrogen sorption and cannot be used for the removal of other gaseous species.
It is an object of the present invention, therefore, to provide an improved composite material capable of continuous hydrogen sorption without a need for an activation treatment, as well as to provide a method for the production of such a material.
The present invention provides a composite material capable of continuously sorbing hydrogen without requiring an activation treatment. The composite material comprises a plurality of particles of a non-evaporable getter (NEG) material that are coated over at least about 10% of their surfaces with a deposit of one or more species selected from the group consisting of palladium, palladium oxide, palladium-silver alloys containing up to about 30% atomic percent silver, and compounds of palladium and the getter material. The NEG material particle size is less than about 500 xcexcm, and more preferably between about 20 xcexcm and about 125 xcexcm, and the NEG material is selected from the group consisting of:
Zr, Ti, Nb, Ta, and V metals;
Zr alloyed with either Ti, Cr, Mn, Fe, Co, Ni, Al, Cu, Sn, Si, Y, La, any of the rare earth elements, or mixtures thereof;
Ti alloyed with either Zr, Cr, Mn, Fe, Co, Ni, Al, Cu, Sn, Si, Y, La, any of the rare earth elements, or mixtures thereof; and
any mixture of the aforementioned metals and alloys.
The NEG material is more preferably selected from the group consisting of:
Tixe2x80x94V alloys, Zrxe2x80x94V alloys, Zrxe2x80x94Al alloys, Zrxe2x80x94Fe alloys, Zrxe2x80x94Ni alloys, Tixe2x80x94Vxe2x80x94Mn alloys, Zrxe2x80x94Mnxe2x80x94Fe alloys, Zrxe2x80x94Vxe2x80x94Fe alloys, Zrxe2x80x94Nixe2x80x94Axe2x80x94M alloys, and Zrxe2x80x94Coxe2x80x94A alloys, where A indicates Y, La, any of the rare earth elements, or mixtures thereof, and M indicates Co, Cu, Fe, Al, Sn, Ti, Si, or mixtures thereof. A particular getter metal, alloy, or mixture thereof can be chosen for a particular service environment, for example, in order to maximize the hydrogen sorption capacity per unit volume. Alternately, a getter material can be selected for its ability to selectively sorb one or more unwanted gas species from the particular service environment. Or, a getter material can be selected for a desirably high melting point necessary to withstand the temperature of a service environment.
The palladium coating thickness is preferably less than about 5 xcexcm and may cover as much as about 100% of the surfaces of the NEG particles. Complete coverage is desirable for those applications where only hydrogen needs to be sorbed. In those applications where one or more gas species other than hydrogen are sought to be sorbed, coverages between about 10% and about 90%, and more preferably between about 25% and about 75% of the particles"" surfaces are coated with the palladium coating. The choice of a particular amount of coverage will include a balance of the need for hydrogen sorption against the need to sorb other gases from the particular service environment.
The composite materials of the present invention can be pressed and sintered to form pellets or other shapes. Pelletized powders can be more desirable for many applications where contamination control is important, and also can be easier to handle in manufacturing environments. The composite materials may also be deposited onto planar and non-planar substrates. Planar and non-planar substrates allow for devices of various geometries and applications, for example, ones having narrow tolerances. They also provide for devices where the quantity of NEG material is high, so that an increased hydrogen sorption capacity is obtained with respect to similar known devices. These materials may also be placed within the evacuated volumes of thermal insulation devices such as thermal bottles, insulated pipes, and dewars. Further, the materials can be used to create getter devices, for example, a container having an opening and filled with a powder of the composite material.
The present invention is also directed to several methods for the preparation of a composite material. In a liquid phase impregnation process a solution of a palladium compound in a solvent is prepared, a plurality of particles of a NEG material are mixed into the solution, the solvent is evaporated to create a dried powder, and the dried powder is thermally treated to leave a palladium coating on the particles. The palladium coating can be palladium metal, palladium oxide, or a mixture of the two. This method is particularly useful for creating uniform thicknesses and complete coverages of the NEG particles.
Other methods involve preparing a thin powder bed of a plurality of particles of a non-evaporable getter material, placing the thin powder bed into an evacuable chamber, evacuating the chamber, and forming a deposit on the particles. One of these methods is a CVD method in which a precursor compound containing palladium is evaporated within the chamber and deposits on the particles in the bed. The particles are then thermally treated to leave a palladium metal or oxide coating. The thermal treatment can also cause the palladium to react with the underlying NEG particle to create a coating that is a palladium compound with one or more of the elements from the NEG material. Another method is an evaporative method in which a wire of a palladium compound is heated within the chamber to cause a portion of the wire to evaporate and deposit on the NEG particles. A third method sputters a target of a palladium compound within the chamber to cause a portion of the target to deposit on the particles in the bed. These methods are particularly useful for creating partial coverages of the NEG particles. Agitating the powder bed during any of these processes can further increase the degree of coverage up to about 100%.
These and other aspects and advantages of the present invention will become more apparent when the detailed description below is read in conjunction with the accompanying drawings.